Authors

Abstract

The impact of environmental heat stress can be evaluated using a physiological strain index (PSI) that incorporates rectal temperature (T­re) and heart rate (HR) of a subject during exercise. Athletes interested in the performance benefits of heat acclimation often over-dress during exercise in temperate environments in an attempt to simulate heat stress, but it is unknown whether this creates a similar level of physiological strain as exercise in the heat. PURPOSE: To compare the PSI of athletes exercising in a HOT (40C; 30% RH) environment and with excess clothing in a COOL (15C; 50% RH) environment. METHODS: Eight endurance trained athletes were studied (5M, 3F; Aged 23 ± 7 years; VO2max 59.8 ± 10.2 ml/kg/min) during one hour of running at 50% of their VO2max in HOT and COOL environments. In the HOT trial, the clothing was minimal (singlet and shorts). In the COOL trial, multiple insulative and vapor-impermeable layers were worn to impose similar themoregulatory strain as the HOT trial. Tre and HR were recorded at 5 minute intervals and used to calculate PSI = 5(Tret− Tre0) ⋅ (39.5 − Tre0)−1+ 5(HRt − HR0) ⋅ (180 − HR0)−1. Mean PSI was compared using paired sample t-tests, and PSI values were additionally compared between trials using an a priori zone of indifference of ±1. RESULTS: PSI rose over time in both environments (HOT: 2.95 to 8.71; COOL: 2.80 to 7.25) Mean PSI was higher in HOT compared with COOL (6.00 ± 0.95 vs 5.16 ± 1.10; p=.042). When comparing mean PSI between HOT and COOL, 5 of 8 subjects tested were within the zone of indifference. The 3 subjects that were outside of the zone had the highest initial HR of all the subjects tested. CONCLUSIONS:These preliminary data suggest that overdressing during a bout of exercise in a temperate environment results in significant increases in PSI, but that the ensemble used did not fully match the HOT condition. By adequately overdressing, athletes may be able to mimic heat stress and potentially obtain the benefits of heat acclimation in a cooler environment.